The present invention relates to the art of rotational speed control. It finds particular application in conjunction with controlling the starting and acceleration of rotating anodes in x-ray tubes and will be described with particular reference thereto. However, it is to be appreciated, that the invention may also find application in other fields.
Conventionally, x-ray tubes have a cathode and an anode. The cathode is typically a filament which is heated with a filament current to a sufficient temperature that a cloud of electrons is boiled off. A high x-ray tube voltage or kV is applied between the cathode and anode to cause a flow of the electrons, a tube current, from the cathode to the anode. The electron flow heats the anode to very high temperatures, near its melting point, such that x-rays are generated. To prevent the anode from becoming thermally damaged in high output tubes, the anode is rotated. Commonly, the anode is a disk which is mounted to an inductive rotor. In very high output x-ray tubes, the anode is often 10 or 15 centimeters in diameter. The rotor, its mounting bearings, the anode, and cathode are all sealed within the x-ray tube envelope. Run and phase windings are mounted outside the x-ray tube envelope adjacent the rotor to provide motive rotational forces thereto. In this manner, a non-synchronous type motor is created, which motor is relatively inefficient due to the spacing limitations between the run and phase windings and the rotor imposed by the x-ray tube envelope.
The large anode and relatively high operational speeds tend to cause bearing degradation. Because the bearings are sealed within the x-ray tube envelope, it is important to preserve the bearings for the life of the tube. Accordingly, the rotating anode is commonly rotated at its operating speed for the generation of x-rays and rotated at a slower speed or stopped when no x-rays are being generated. To meet customer demand and to minimize mechanical resonance, the rotor and anode typically accelerate quickly to the operating speed. Typically, an electrical braking force is applied after the x-ray exposure to return the anode more quickly to its idle speed.
Commonly, high speed x-ray tube anode starters use open loop type controls. That is, AC currents of the appropriate phase and frequency to cause rapid acceleration of the rotor are applied to the run and phase windings. In these open loop systems, the laws of electromagnetism are relied upon to assure that the anode actually accelerates as projected.
Other anode starters use closed loop analog circuitry to compare the actual and anticipated rotational speed of the rotor and anode. The speed of the rotor and anode can be determined directly with a tachometer or derived indirectly from the relative phase, frequency, and magnitude of the currents through the run and phase windings. A rotor that is lagging or leading the rotational speed that the phase and frequency of the applied run and phase currents are designed to provide causes measurable alterations in these currents. These measurable differences or analog error signals can control a modulator which in turn controls the AC current pulses applied to the run and phase currents.
Typical prior art rotor speed control circuits are shown in U.S. Pat. No. 4,829,551 issued May 9, 1989 to Messrs, Resnick, Dupuis, and Szabo and U.S. Pat. application Ser. No. 07/489,465 of Rarick. The present invention contemplates a new and improved digital starter speed monitor and control method and apparatus.